The present invention relates to a cathode ray tube and a method of manufacturing a cathode ray tube characterized by very high resolution and performance.
Cathode ray tubes for color reproduction typically employ a plurality of electron guns, each adapted to produce an electron beam directed toward phosphor screen elements of a given color. For example, the screen of the cathode ray tube may comprise a pattern of interspersed red, green and blue phosphor elements or dots each adapted for receiving excitation from one of three electron guns. Interposed between the phosphor screen and the electron gun structure is a so-called shadow mask comprising a thin metal sheet having a multiplicity of perforations positioned to pass the electron beam from one of the guns to the phosphor dots of a given color. For example, the array of apertures in the mask will allow the electron beam from the "red" gun to impinge only upon red phosphor dots while the non-apertured or blank part of the shadow mask casts a "shadow" over the green and blue dots. The "green" and "blue" guns are positioned in relation to the "red" gun, typically in triangular array, such that an electron beam from the "green" gun will strike only green phosphor dots and the electron beam from the "blue" gun will strike only blue phosphor dots.
The color resolution of the color tube is dependent upon the correct manufacture of the shadow mask and the correct alignment of the shadow mask relative to the phosphor elements or dots on the screen under operating conditions. A number of manufacturing methods have been utilized in attempting high color resolution. One method includes mechanical tensioning of a flat shadow mask relative to a heavy metal support frame that is then positioned inside the tube in spaced relation to the screen. However, the mechanical tension heretofore attained has been insufficient by itself to insure color purity and furthermore the metal frame can become displaced with respect to the screen as a result of heat or vibration. These mis-registration problems can be intolerable in very high resolution color tubes used for avionics purposes and the like.
Another method for emplacement of a shadow mask in a color cathode ray tube combines mechanical tensioning with applied heat. The shadow mask may be mechanically stretched and at the same time raised to a high temperature before being secured to a metal frame or to the cathode ray tube itself, while utilizing a shadow mask material having a coefficient of expansion substantially greater than that of the frame or tube to which it is to be attached. Under these circumstances, the shadow mask will contract more than the surrounding frame or envelope with cooling, resulting in higher mask tension and improved performance. As with the purely mechanical approach of the prior art, insufficient tension is applied to the shadow mask in this manner to insure color purity, particularly when the shadow mask may be subjected to high beam currents attendant to high brightness displays. The cathode ray tube, and particularly the shadow mask, often become heated during normal operation to relatively high temperatures, resulting in the reverse of the process under which the tension was achieved. I.e., the application of heat causes greater expansion of the shadow mask than the frame or tube and resultant deterioration in color resolution.
In U.S. Pat. No. 4,069,567 to Schwartz, a cathode ray tube construction is described wherein a shadow mask having a coefficient of thermal expansion substantially higher than the holder or envelope is heated to a higher temperature during manufacture than the holder or envelope whereby a higher tension is achieved after cool down than would result if both envelope and mask were heated to the same temperature. While resulting in higher room temperature tension, the mask in this case is still subject to undue relaxation of the mask when heated by high energy electrons. In any of the foregoing methods, the accuracy of shadow mask registration is insufficient to withstand high vibration effects encountered in aircraft, and to withstand high beam currents used for producing clearly visible color presentations under daylight conditions.